1. Field of the Invention.
This invention generally relates to enhancing the fatigue life of the material around a hole in a structural part by subjecting the hole to a cold expansion process, and more particularly to a method for inspecting a cold expanded (CX) fastener hole to confirm that the hole has been expanded by a desired amount.
2. Description of the Related Art.
Fastener holes drilled into a structural part, which, for example, may be a metal plate of a type used to construct the fuselage or control surfaces of an aircraft, may fail over time from fatigue cracking. In order to prevent this from happening, a process known as cold expansion is often used to condition the material around the holes so that the material will develop a resistance to fatigue stress.
The cold expansion process is usually performed by pulling an oversized mandrel through a fastener hole in accordance with a predetermined schedule. The mandrel compresses the material around the hole beyond the elastic-plastic boundary until plastic deformation occurs and the hole diameter becomes permanently increased by a desired amount. It is the plastic deformation that occurs in the material around the hole that increases its resistance to fatigue cracking.
The cold expansion process, however, has been known to fail if improperly executed. The CX holes in a structural part therefore should be inspected before the part is installed in order to confirm that the holes have been expanded by an amount sufficient to achieve a desired level of fatigue resistance.
It is well known that the plastic deformation that occurs during cold expansion causes a residual compressive circumferential stress distribution to form in the metal surrounding the CX hole. It is also well known that the degree of expansion of a CX hole is proportional to, and thus can be determined by measuring, this residual stress distribution.
Known methods for measuring the residual stress distribution around a CX hole require that a series of diameter measurements be taken at discrete points throughout the length of the hole before and after the expansion process. A statistical analysis is then performed, which involves taking the average difference between the pre- and post-expansion diameter measurements. A particular residual stress distribution around the CX hole is then assumed to exist based on this statistical average.
Residual stress distribution measurements computed in this fashion have proven to be inaccurate for at least two reasons. First, the gauges used to take the hole diameter measurements are of limited accuracy. Any errors associated with these hole diameter readings will translate as errors in the stress distribution measurement.
Second, the statistical analysis used is based on a number of faulty assumptions which further distort the stress distribution measurement. It is assumed, for example, that the diameter of a CX hole is uniform throughout its length, but this is clearly not so. In a vast majority of cases, the mandrel used to perform the cold expansion produces a hole with a non-uniform diameter. A stress distribution measurement based on the assumption that the hole was of uniform diameter therefore will be inaccurate in cases where the hole in fact has a non-uniform diameter.
Prior art methods for measuring the expansion of a CX hole are undesirable for two other reasons. One, they are slow to implement primarily because of the care which must be taken in making the required hole diameter measurements. And two, prior art methods for measuring the degree of expansion of a CX hole can only be employed before the structural part is installed.
Based on the foregoing discussion, it is clear that a need exists for a method of measuring the degree of expansion of a CX hole which is free from the drawbacks associated with prior art cold expansion measurement techniques, and preferably one which is faster, more accurate, and simpler to implement compared with its prior art counterparts.